The invention relates to the combination of a electrical motor and generator and more particularly, to a single device performs both starter motor and alternator functions in a vehicular engine system.
In general, an electromagnetic machine can be operated either as a motor or as a generator depending respectively upon whether power is delivered to the unit from an external source of electrical energy or whether the unit is mechanically driven by an external source of mechanical energy, such as an internal combustion engine.
The subject invention comprises a structure particularly adapted for automotive applications which permits the combination of the starter motor function and the generator (generic DC or AC) or alternator (AC) function in a unique package to take advantage of the motor/generator characteristics described above. The subject invention lends itself to being positioned between the internal combustion engine and the transmission in the drive train of an automobile, making use of either the fly wheel or the torque convertor as part of the system. This is in contrast to the traditional location of a separate conventional automotive starter motor which is momentarily connected to the fly wheel on the engine during the cranking or starting cycle, and the traditional location of a separate generator or alternator which is typically belt driven from the crankshaft of the engine. Since these functions of the starter and alternator are combined, the unit arranged in accordance with the principles of the invention may be located in line between the engine and transmission, thereby eliminating the need for being belt or gear driven and to take advantage of the fact that only one motor/generator unit will be used in place of two units, as in present conventional arrangements.
Various approaches in producing a dual purpose starter generator machine for use on motor vehicles have been developed since the early 1900's. For example, U.S. Pat. No. 1,250,718 to Turbanyne, issued Dec. 18, 1917, discloses a DC motor/generator having a rotating armature that is ring wound. When operated as a DC motor, DC current is supplied to the rotor through conventional commutator brushes.
Another starter generator is disclosed in U.S. Pat. No. 1,325,677 to Midgley, issued Dec. 23, 1919. In this design, a conventional DC machine having a wound rotor is fitted with four brushes on the commutator ring. One of these brushes is movable away from the commutator. Movement of the movable brush serves to engage or disengage an automatic circuit as a voltage regulation device when the machine is operated as a generator. When operated as a DC motor, the movable brush effectively disengages the automatic circuit by connecting the circuit across two brushes of the same polarity.
Another example of a starter generator machine is disclosed in U.S. Pat. No. 2,184,236 to Heintz, issued on Dec. 19, 1939. In this machine, in addition to conventional slip rings and brushes for energizing the windings on the rotor during generator and motor operation, the rotor is fitted with rotatable brushes. These rotatable brushes are in engagement with a stationary commutator which feeds low voltage direct current to the stator windings during the engine cranking operation. The rotating brushes are moved out of engagement with the commutator by centrifugal force as the engine crankshaft is accelerated. Thus, in this design, the rotatable brushes provide a rotating stator magnetic field for operation of the device as a motor. When operating as a generator, the Heintz device produces alternating current.
In a more recent starter motor alternator disclosed in U.S. Pat. No. 4,219,739 to Greenwell, issued Aug. 26, 1980, the main rotor winding is connected in series with the main stator winding. In addition, the exciter armature winding is on the rotor and the exciter field winding is on the stator. During starter motor operation, the main rotor winding is connected in series with the starter field winding through a commutator and conventional DC brushes. During alternator operation, the brushes are lifted off the commutator, and the exciter armature winding slip rings are connected to the main rotor winding.
In all of the above examples, external current is fed through a commutator to the windings on the rotor. The current carried by the conductor in the magnetic field produces a torque which causes rotation of the machine as a motor. When operated as a generator or alternator, current is once again fed through a commutator or slip rings to windings on the rotor to provide excitation. These dual purpose motor generator sets have a variety of disadvantages. In any conventional dual purpose machine, certain sacrifices must be made in order to accommodate both generator and motor functions in a single device. For example, previous conventional motor generator designs for use in a motor vehicle such as an automobile or an aircraft have a low power to size ratio, are relatively expensive, and have a high length to diameter ratio. It has therefore been impractical to develop a combined motor generator design for use in automotive engine systems.
The dual purpose machine concept has primarily been utilized in aircraft. However, these machines are extremely complex to manufacture with resultant high cost. Because of the power requirements, overall size and complexity of a conventional motor generator or dual starter motor alternator of conventional design, automotive vehicles utilize separate starter motors and alternators in past practice.
The disadvantages of conventional starter motor designs include very high noise during operation, a low electri-mechanical efficiency, relatively large size requirements, high motor weight and battery size requirements and low reliability. In addition, the necessity for providing a separate alternator increases the overall space allocation requirements for the alternator and starter motor functions.
A known approach to providing a combined starter motor/alternator system is set forth in U.S. Pat. No. 4,862,009 to King. The King patent discloses a combined starter/alternator which may be coupled directly to the vehicle drive train and which may take the form of a permanent magnet motor/alternator machine. The King system utilizes a three phase invertor in the starter mode with an electronic commutation system which uses Hall-effect sensors. Torque multiplication is achieved via a modified planetary gear assembly.
One prior approach to providing motor/generator units having a flat, non-magnetic ironless stator which could be mounted to the drive train of the vehicle is disclosed in U.S. Pat. No. 5,001,412 to Crall et al., issued Mar. 19, 1991, and assigned to the same assignee as the instant invention. However, it has been found that under high speed engine operating conditions, eddy current losses significantly increase in the flat planar windings utilized in the stator of the Crall et al. prior invention.
There also is seen to be a need for a drive circuit in the motor operating mode providing improved protection for the switching devices used therein when subjected to high reverse bias voltages resulting from high speed engine operation with the electrical machine operating in the alternator mode.